You can't just walk in and out carrying 12kg bars. Also I don't think you can just buy a bar of tungsten, you gotta smelt it, coat with gold, not trivial. That

You can't just walk in and out carrying 12kg bars. Also I don't think you can just buy a bar of tungsten, you gotta smelt it, coat with gold, not trivial. That kind of operation would involve a lot of cooperating people. You also need to convert gold to dollars which might not be as trivial as you think. So maybe that happens, but it's a lot more complicated. And, of course, there are measures against that happening.